#include<stdio.h>
#include<math.h>
int main()
{
    //freopen("in.txt","r",stdin);
    float a,b,c;
    scanf("%f %f %f",&a,&b,&c);
    float p = (a + b + c)/2;
    printf("%.1f",sqrt(p*(p-a)*(p-b)*(p-c)));
}